1. Field of the Invention
The present invention relates to a thin film transistor, and, more particularly, to a thin film transistor including an N-type compound semiconductor as a semiconductor layer of the thin film transistor.
2. Description of Related Art
A thin film transistor used for a conventional organic light-emitting display device can be a thin film transistor that includes amorphous silicon or polysilicon as a semiconductor layer of the thin film transistor.
However, if the semiconductor layer is formed of amorphous silicon, it is difficult to use the semiconductor layer as a drive circuit of a display panel which requires a high operating speed due to the low mobility of the amorphous silicon. By contrast, polysilicon has a high mobility, but an additional compensation circuit should be provided because its threshold voltage is not uniform.
Also, the thin film transistor including material such as amorphous or polysilicon as the semiconductor layer has a problem in that its transistor characteristics are deteriorated due to leakage of electric current generated by the light irradiation.
Accordingly, in order to solve the above problems, there have been attempts to develop a compound semiconductor. For example, FIG. 1 illustrates a thin film transistor using a compound semiconductor that includes ZnO or ZnO as a semiconductor layer of the thin film transistor.
Referring to FIG. 1, the thin film transistor includes a source electrode 20a and a drain electrode 20b formed on an insulating substrate S; a ZnO layer 4 arranged to be in contact with the source and drain electrode 20a, 20b; and a gate insulator 5 and a gate electrode 6 laminated onto the ZnO layer 4.
Here, in FIG. 1, ZnO or a compound semiconductor including the ZnO has a band gap of 3.4, and therefore it is expected that the thin film transistor has an effect that the leakage of an electric current is not increased by the visible light absorption because the ZnO or the compound semiconductor does not absorb the visible light due to the fact that its band gap is higher than a light energy of a visible region.
However, it is common that the ZnO or the compound semiconductor including the ZnO has N-type conductivity due to the oxygen shortage, whereas the organic light-emitting display device uses a P-type driving element.
For example, FIG. 2 illustrates an organic light-emitting display device using a P-type driving element. Referring to FIG. 2, the organic light-emitting display device includes a P-type driving thin film transistor including a gate electrode 40, a source electrode 50a, a drain electrode 50b and a P-type semiconductor layer 60, which are all formed on a substrate 30.
In this case, the source electrode 50a of the driving thin film transistor is connected to a common power pressure line (ELVDD) 96, the drain electrode 50b is connected to an anode electrode 80 of the organic light-emitting diode (OLED), and an organic light-emitting layer 90 is formed on the anode electrode 80, and a cathode electrode 92 formed on the organic light-emitting layer 90 is connected to a second power pressure line 97 to drive an organic light-emitting display device. In addition, an insulator 93, a pixel definition layer 94, a sealing material 98 and an encapsulation substrate 99 are also shown in FIG. 2.
Accordingly, in order to use a driving thin film transistor using an N-type compound semiconductor as a semiconductor layer in the organic light-emitting display device, an anode electrode and a cathode electrode of the organic light-emitting diode have an inverted structure, and therefore there is a need to improve the structure of the organic light-emitting display device.